A pilot study has demonstrated that the virulence for man of live influenza viruses produced by recombination is closely related to the pathological effect and growth of these viruses in organ cultures of human embryonic trachea. The viruses which were virulent for man caused profound damage to the ciliated epithelium of the trachea while growing to high titre, where as the reverse was true for viruses attenuated for man. The immediate objective of this study is to quantitate the effects and growth of several live influenza virus vaccine candidates produced both by recombination and by other methods of attenuation. Based on the behavior in organ cultures of human and ferret trachea, the viruses will be ranked in order of virulence and results compared to those obtained in human volunteers. The basic contribution of this work will be that this organ culture laboratory marker will finally obviate the need for human volunteers currently used for initial screening of various influenza viruses selected as live vaccine candidates, and provide a long awaited method for ranking influenza viruses by relative virulence.